Carbon-carbon and/or metal-carbon fiber composite heat spreader

ABSTRACT

A heat spreader, comprised of a plurality of carbon fibers oriented in a plurality of directions, with a carbon or metal matrix material dispersed about the fibers, is described. The carbon fibers facilitate the spreading of heat away from the smaller semiconductor device and up to a larger heat removal device, such as a heat sink.

The present patent application is a divisional of application Ser. No.10/991,621 filed Nov. 17, 2004, which is now abandoned, which is adivisional of application Ser. No. 09/955,889 filed Sep. 18, 2001, whichis issued as U.S. Pat. No. 6,837,306, which is a divisional ofapplication Ser. No. 09/670,923 filed Sep. 29, 2000 which is issued asU.S. Pat. No. 6,469,381.

BACKGROUND

1. Field of the Invention

This invention relates to semiconductor manufacturing technologygenerally, and more specifically, to heat spreader technology for heatdissipation in a semiconductor package.

2. Description of the Related Art

There is a trend toward increasing the number of functions built into agiven integrated circuit (also referred to herein as a device). Thisresults in an increasing density of circuits in the device. Along withthe increased circuit density, there is always a desire to increase thedata processing rate; therefore, the clock speed of the device isincreased as well. As the density of circuits and the clock speedincrease, the amount of heat generated by the device increases.Unfortunately, device reliability and performance will decrease as theamount of heat that the device is exposed to increases. Therefore, it iscritical that there be an efficient heat-removal system associated withintegrated circuits.

FIG. 1 illustrates a typical integrated circuit and associatedpackaging. There are a number of methods for removing heat fromintegrated circuits 103, including active methods, such as fans orrecirculated coolants (not shown), or passive methods, such as heatsinks 107 and heat spreaders 105. Because of the decreasing device 103size, there is usually a need to evenly distribute heat generated by thesmall device 103 to the larger heat sink 107 to eliminate “hot spots” inthe device. This is the function of heat spreaders 105. Heat spreadersare coupled to the integrated circuit 103 through the use of a thermallyconductive material 104. These thermal interface materials 104, such asgel or grease containing metal particles to improve heat conduction, areapplied in between the device 103 and the heat spreading structure 105to improve the heat transfer from the integrated circuit 103 to the heatspreader 105. Typically, the heat spreading structure 105 will beconstructed either of a ceramic material or a metal, such as aluminum orcopper. Aluminum is preferred from a cost standpoint, as it is easy andcheap to manufacture; however, as the heat load that needs to betransferred increases, copper becomes the metal of choice because of itssuperior heat transfer characteristics (the thermal conductivity for Alis ˜250 W/m·K vs. ˜395 W/m·K for Cu.) There will typically be acontiguous wall 106 around the periphery of the heat spreader, whichserves as a point of attachment and support between the substrate 101and the spreader 105. There is often a heat sink 107 attached to theheat spreader 105, to allow for the greater cooling capacity associatedwith the high surface area of the heat sink 107.

With increased heat dissipation requirements, it has become necessary toimprove heat spreader 105 and/or heat sink 107 performance. Whileimproving heat sink performance through active cooling methods such asfans or recirculated liquids works well, there are a number ofdisadvantages associated with this solution, including bulkiness, costand noise.

A second method for increasing heat dissipation capacity for integratedcircuit packaging is through improvement of heat spreader performance.Current heat spreader materials allow for heat conduction in the rangeof 80-400 W/m-°K. FIG. 2 illustrates one method of increasing the rateof heat conduction in heat spreaders 201 a, 201 b. FIG. 2 a shows a topview of a heat spreader 201 a while FIG. 2 b illustrates a cross-sectionof the same heat spreader 201 b. Composites using layers of highlyconductive carbon fibers 202 a, 202 b impregnated with carbon resin ormetals 203 a, 203 b are known to be very effective conductors of heat.These materials also offer the added advantage of lighter weight ascompared to the present materials (e.g. a density of 5.9 g/cc for Cumatrix composite versus 8.9 g/cc for copper), decreasing packagingweight, shipping cost and offering ergonomic advantages formanufacturing personnel. However, these materials have suffered from thedisadvantage of being anisotropically oriented in their heat flow, thusthey are typically highly conductive (>500 W/m-°K) in only onedirection. The direction of heat conduction follows the longitudinalorientation of carbon fibers, therefore the unidirectional heat flow isa result of the majority of the fibers in the composite being orientedin one direction. The aforementioned advantages are often outweighed bythe disadvantage of poor heat conduction in both the second horizontaland the vertical directions.

Therefore, what is needed is an apparatus for increasing the rate ofheat transfer in all three directions, allowing the rapid dissipation ofheat through the heat spreader and to the heat sink.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example, and notlimitation, in the Figures of the accompanying drawings in which:

FIG. 1 shows a prior art integrated circuit package design.

FIG. 2 a shows a prior art heat spreader design.

FIG. 2 b shows a cross-section of a prior art heat spreader design.

FIG. 3 shows a cross section of an integrated circuit package containingan embodiment of a heat spreader using carbon fibers to spread heat inmultiple dimensions.

FIG. 4 a shows a top view of an embodiment of a heat spreader usingcarbon fibers to spread heat in multiple dimensions

FIG. 4 b shows a cross-section of an embodiment of a heat spreader usingcarbon fibers to spread heat in multiple dimensions

FIG. 5 shows a different embodiment of a heat spreader using carbonfibers to spread heat in multiple dimensions.

FIG. 6 shows an embodiment of a heat spreader using carbon fibers tospread heat in multiple dimensions where there are thermal interfacelayers on the top and bottom of the heat spreader to modify heatdissipation efficiency.

FIG. 7 shows an embodiment of a heat spreader using carbon fibers tospread heat in multiple dimensions that contains chopped fibers inbetween the horizontal fiber layers.

FIG. 8 a shows top and side views of an embodiment of a heat spreaderusing carbon fibers to spread heat in multiple dimensions that hasattached standoffs.

FIG. 8 b shows top and side views of differing embodiments of standoffsthat can be used on the heat spreader of FIG. 8 a.

FIG. 9 shows an integrated circuit package, containing an embodiment ofa heat spreader using carbon fibers to spread heat in multipledimensions, which contains a plurality of integrated circuits.

DETAILED DESCRIPTION

An apparatus for increasing the rate of heat flow through a heatspreader is described. In the following description, numerous specificdetails are set forth such as material types, dimensions, etc., in orderto provide a thorough understanding of the present invention. However,it will be obvious to one of skill in the art that the invention may bepracticed without these specific details. In other instances, well knownelements and processing techniques have not been shown in particulardetail in order to avoid unnecessarily obscuring the present invention.

A heat spreader, comprised of a plurality of carbon fibers oriented in aplurality of directions, with a carbon or metal matrix materialdispersed about the fibers, is described. The carbon fibers facilitatethe spreading of heat away from the smaller semiconductor device and upto a larger heat removal device, such as a heat sink.

This discussion will mainly be limited to those needs associated withremoving heat from the backside of a flip chip that is housed within aSMT or INT package. It will be recognized, however, that such focus isfor descriptive purposes only and that the apparatus and methods of thepresent invention are applicable to other types of electronic devicesand other types of packaging.

FIG. 3 illustrates a cross-section view of a semiconductor package inone embodiment of the present invention. The package includes asubstrate 301 having a semiconductor device 303 mounted on a top surfaceof the substrate 301. In one embodiment the substrate 301 is a printedcircuit board. In another embodiment, the substrate 301 may be adifferent material, such as silicon or ceramic.

In one embodiment, the semiconductor device 303 is mechanically andelectrically coupled to the top surface of the substrate via a pluralityof solder bump connections 302. In an embodiment the gap may be filledwith an epoxy underfill material (not shown). The substrate 301 containsat least one wiring layer (not shown) that electrically connects thedevice to pins or balls located along the bottom surface of thesubstrate 301.

In accordance with the present invention, a composite heat spreader 305is thermally coupled to the bottom of the flip chip structure 302, 303through a compliant heat-transfer medium 304. In one embodiment, theheat transfer medium is thermal grease. In another embodiment, gel orother proprietary formulations may be used.

The heat spreader is further attached to the substrate using a sealantmaterial 307. The sealant material 307 surrounds the device 303 andfills the gap between the substrate 301 and the heat spreader 305,forming a completely enclosed cavity containing the device 303. The useof the sealant material 307 allows for a more flexible bond between thesubstrate 301 and the heat spreader 305. In one embodiment the sealantmaterial may be silicone or other proprietary sealant material. Theflexible bond may help to compensate for differing coefficients ofthermal expansion (CE) between the heat spreader and the substrate,resulting in a more consistent heat conduction pathway. A secondadvantage of the current embodiment is that the sealant is much lighterin weight compared to the metal used in the prior art contiguous wall(See FIG. 1, 106) design, resulting in a lighter package.

Next, a heat sink 306 is attached to the heat spreader 305 using athermal interface material 308. In one embodiment, the thermal interfacematerial 308 is thermal grease. The heat sink 306 should allow for themore rapid dissipation of heat due to increased surface area forcooling, as discussed in the Background section above.

FIGS. 4 a (top view) and 4 b (cross-sectional view) further illustratethe heat spreader of FIG. 3. The heat spreader 305 a; 305 b is formedfrom a composite material comprised either of carbon fibers 402 a, 404a, 405 a, 402 b, 404 b, 405 b impregnated with a resin material 403 a,403 b (referred to as a carbon/carbon composite) or carbon fibers 402 a,404 a, 405 a, 402 b, 404 b, 405 b impregnated with a metal or metalalloy 403 a, 403 b (referred to as a metal/carbon composite). In oneembodiment of the present invention, a carbon/copper composite materialis used. However, in another embodiment, the composite may use adifferent thermally conductive matrix metal, such as aluminum ormagnesium, a metal alloy, a ceramic, such as silicon carbide, or anorganic material, such as resin.

One factor in the choice of what composite material to use may be whatmaterial had been used previously in the packaging process. Matching themetal/composite material with the previous heat spreader material mayallow the use of the same adhesive or thermal grease system aspreviously used, thus simplifying the conversion process from one heatspreader material to another. A second factor to consider in choosingthe type of composite material to use is the CTE of the substratematerial. It may be possible to better match the CTE of the heatspreader with that of the substrate, allowing for the production of amore reliable package.

In the present embodiment, the heat spreader contains horizontal layersof fiber bundles 4022 a, 404 a and 402 a, 404 b, consisting of twoperpendicular sets of fiber bundles woven into a sheet, oriented in thex-y plane of the apparatus. These woven fiber bundles facilitate heatconduction in the x-y plane. In addition, there is a second set of fiberbundles 405 a, 405 b, oriented substantially perpendicular to the firstset. The substantially perpendicular fiber bundles facilitate theconduction of heat in the z-direction.

In the present embodiment, the fiber bundles are comprised ofapproximately 1000 individual carbon fibers twisted into a fiber bundle.These bundles of fibers are then woven into a sheet. The weave in thepresent invention should be balanced, to produce a flatter heatspreader. The weave may be balanced by attempting to ensure that thefiber throughout the x-y plane of the woven mat have substantially thesame number of downward stitches as up. In one embodiment, theindividual carbon fibers have a diameter of approximately 10 microns,with a density of about 2.2 g/cc. In this embodiment, the fibers mayhave a thermal conductivity as high as 1000 W/m·K. One example ofcommercially available carbon fiber is Amoco K1100 2K™. While carbonfibers are discussed in this embodiment, other types of highlyconductive (>500 W/m·K) fibers or wires, based on materials such aspolymers, metals or ceramics, may work in the present invention. In adifferent embodiment, the fibers may have very different physical anddimensional properties, and the above thermal and physical propertiesshould not be construed as limiting the properties of the fibrousmaterials used.

The woven fiber sheets are then impregnated with a metal, metal alloy,carbon or ceramic matrix material, as discussed above. The matrixmaterial may be dispersed about the woven sheets using a number ofdifferent methods. In this embodiment, a compression molding method isused. In another embodiment, injection molding or any of a multitude ofmolding processes practiced in the art may be used. The molded materialmay be allowed to cure, and then, if necessary, could be cut to thecorrect dimensions. In an embodiment, a laser may be used to cut thecomposite material. In another embodiment, a mechanical means, such as asaw or mill, may be used.

The heat spreader in the current invention provides for betterconduction of heat in the z-direction, with a possible thermalconductivity in the range of 500-1000 W/m·K. In the present embodiment,a finned heat sink can be attached to the top surface of the heatspreader. Through the use fibers oriented in the z-direction, heat canbe conducted up through the heat spreader to the heat sink, and the heatcan be dissipated to the surrounding environment, with cooling for thesink provided by the surrounding air or an active cooling method, asdiscussed in the Background section.

In addition, the fibers oriented in the x-y plane 402 a, 402 b, 403 aand 403 b allow the heat to dissipate radially, thus preventing theformation of localized hot spots. Localized heating decreases the areaavailable for heat transfer, which decreases the overall heat flux fromthe device. The fibers in the x-y plane 402 a, 402 b, 403 a and 403 ballow the conducted heat to rapidly dissipate from the relatively smallcontact area in the point of attachment of the device to the heatspreader, over virtually the entire (larger) area of the heat spreader.This means that heat can be removed much more efficiently.

FIG. 5 illustrates that the orientation of the fibers in the z-directionmay differ from the essentially perpendicular orientation above. In thisembodiment, the fibers in the z-axis 502 may be oriented atapproximately +/−30 degrees from fibers in the x-y plane 503. It is tobe understood, however, that the relative orientation in the z-directionmay comprise any angle between 0 and +/−90 degrees from the x-y plane.

In a third embodiment of the invention, it may be desirable to include ahigher density thermal interface layer on the top and bottom surface ofthe heat spreader, as shown in FIG. 6. In this embodiment, the thermalinterface layer 601 could comprise the same carbon fiber material usedin the above invention 603, only with an increased fiber density fromthat used in the main body of the heat spreader 603. In an embodiment,the fiber density in the thermal interface layer 601 may be four timesthat of the main body 603. While this embodiment may use the same fiberas the above embodiments, it is understood that other thermallyconductive materials may also be used as the thermal interface material,including non-composite materials. In addition, the higher-densitycarbon fiber layer may be comprised of chopped fibers, as discussed inFIG. 7 below. In an embodiment, the increased fiber density 601 mayallow for even greater heat conduction capability, and may be used tomore rapidly dissipate heat in the x-y direction from hot spots in thedevice 303/ heat spreader 305 interface region, in addition to allowingmore rapid conduction from the heat spreader 303 to the heat sink (notshown) through the top layer 602.

In another embodiment of the present invention, the fibers predominantlyoriented in the z-direction are replaced with chopped fibers. In anembodiment, the chopped fibers are comprised of carbon fiber that hasbeen broken or cut, using a mechanical means, into segments less than0.5 mm long. However, the length of the chopped fiber may varyconsiderably from this length, and the aforementioned dimensions shouldnot be construed as limiting the allowable chopped fiber length.

In addition, another embodiment may use some other type of highlyconductive fibrous material, rather than carbon fiber, as discussedabove in FIG. 4. In this embodiment, shown in FIG. 7, the chopped fibers701 are placed in between the woven sheets 702 that are oriented in thex-y plane, forming a layered structure. The chopped fiber 701 and thewoven sheets 702 are then impregnated with a carbon or metal-basedmatrix material 703, to form a composite. The chopped fibers 701 may aidin increasing thermal conductivity in the z-axis, much like the orientedfibers discussed in previous embodiments. The use of chopped fibers 701may allow the manufacture of a heat spreader with many of the advantagesdiscussed above at a reduced cost. In a further embodiment, the wovensheets 702 may be eliminated all together, and chopped fibers 701 orother type of carbon material, such as carbon flakes, dispersedthroughout the heat spreader may be used to facilitate conductionthrough the matrix.

Recall from the discussion of FIG. 3 that the heat spreader 305 isattached to the substrate 301 using a sealant material 307. This allowedfor a more flexible and lighter weight point of attachment between thetwo structures. Sometimes, however, a more rigid package may be desired.FIGS. 8 a and 8 b illustrate how a more rigid structure may be achievedwith the present invention. In this embodiment, a plurality of legs 802a are added to the heat spreader 305 a to serve as additional supportand points of attachment when bonding the heat spreader 305 a to thesubstrate (301 in FIG. 3.) In this illustration, four cylindrical legsare shown. It is to be understood that the number of legs, and theirshape, may vary from application to application. Referring to FIG. 8 b,other shapes may include, but are not limited to, rectangular legs 801b, contiguous walls 802 b, noncontiguous walls 803 b, rectangular legswith holes 804 b or rectangular legs with feet 805 b. The relationshipof the substrate to these structures is shown by 806 b. In anembodiment, the above structures are approximately 0.63 mm tall. Itshould be understood, however, that the height of these structures willvary with different applications, and that the above dimension shouldnot be construed as limiting the size of the structures.

Referring again to FIG. 8A, the legs 802 a may be constructed using avariety of materials and methods. In one embodiment, they areconstructed of a polymeric material. One example of such as a materialis a high modulus epoxy. One method of manufacturing legs using highmodulus epoxies would be injection molding. However, there are amultitude of other methods that may be used for forming the legs,depending on the material used. These include, but are not limited to:Machining, liquid resin molding and thermoforming.

The legs 802 a may be attached to the heat spreader 305 a through abonding process. Examples of bonding types may include attachment withan adhesive, such as an epoxy, or soldering. Depending on the type ofmaterial used for the legs and the type of bonding process, it may benecessary to roughen the surface of the heat spreader at the point ofattachment, to increase the strength of the foot/heat spreader bond.Although there are a multitude of methods that can be used, examples oftechniques used for roughening may include mechanical means, or throughlaser marking.

While the previous embodiments have focused on flip chip packagescontaining a single device, the present invention could also be used forpackaging substrates with multiple integrated circuit devices attached.As shown in FIG. 9, these packages would have a configuration similar tothat of a single chip package, containing multiple devices 303 attachedto the substrate 301 through ball-grid arrays 302 and thermally coupledto the carbon/carbon or metal/carbon heat spreader 305 using a compliantheat-transfer medium 304. The heat spreader 305 is coupled to thesubstrate using either a sealing material 307 or through a combinationof sealing material and legs, as discussed in FIG. 8. The heat spreaderwill be further attached to a heat sink 306 to facilitate the removal ofheat from the heat spreader 305. All of the aforementioned embodimentswith regard to heat spreader construction may also apply to themulti-chip configuration.

Thus, what has been described is an apparatus for spreading heat removedfrom the backside of a packaged semiconductor device. In the foregoingdetailed description, the apparatus of the present invention has beendescribed with reference to specific exemplary embodiments thereof. Itwill, however, be evident that various modifications and changes may bemade thereto without departing from the broader spirit and scope of thepresent invention. The present specification and figures are accordinglyto be regarded as illustrative rather than restrictive.

1. A method of forming a heat spreader, comprising: placing a firstplurality of woven fibers into a mold to form a first layer, the firstplurality of fibers disposed in mostly horizontal directions; placing asecond plurality of fibers into the mold above the first plurality ofwoven fibers to form a second layer above the first layer, the secondplurality of fibers disposed in a mostly vertical direction, wherein thefirst layer has fibers orienting in mostly horizontal directions and thesecond layer has fibers orienting in a mostly vertical direction;disposing a heat conductive material around the first plurality of wovenfibers and the second plurality of woven fibers; and curing the heatconductive material.
 2. The method of claim 1, wherein the fibers arecomprised of carbon.
 3. The method of claim 1, wherein the fibers arechopped.
 4. The method of claim I, wherein disposing the heat conductivematerial comprises compression molding.
 5. The method of claim 1,wherein disposing the heat conductive material comprises injectionmolding.
 6. The method of claim 1, further comprising cutting the heatconductive material, the first plurality of woven fibers and the secondplurality of fibers to form the heat spreader.
 7. The method of claim 1,wherein the first plurality of fibers has a higher fiber density thanthe second plurality of fibers.
 8. The method of claim 1, wherein thefirst plurality of fibers and the second plurality of fibers havesimilar fiber densities.
 9. The method of claim 1, wherein the fiberdensity of the first plurality of fibers has a fiber density four timesgreater than the fiber density of the second plurality of fibers. 10.The method of claim 1, wherein the mold is comprised of a compositematerial.
 11. The method of claim 1, further comprising attaching theheat spreader to a heat sink.